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Clinical Microbiology Reviews, April 2008, p. 305-333, Vol. 21, No. 2
0893-8512/08/$08.00+0 doi:10.1128/CMR.00060-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.
Cavitary Pulmonary Disease
Division of Infectious Diseases and International Health, Duke University Medical Center, Durham, North Carolina
SUMMARY INTRODUCTION WHAT MAKES A CAVITY? Pathophysiology of Cavities CHEST IMAGING TO DETECT CAVITIES Characteristics of Cavities Used for Differential Diagnosis NONINFECTIOUS DISEASES ASSOCIATED WITH LUNG CAVITIES Malignancies Rheumatologic Diseases Miscellaneous Diseases Associated with Cavities INFECTIONS ASSOCIATED WITH LUNG CAVITIES Common Bacterial Infections Necrotizing pneumonias and lung abscesses. Septic pulmonary emboli. Uncommon Bacterial Infections Actinomycosis. Nocardia. Melioidosis. Rhodococcus. Mycobacterial Infections Mycobacterium tuberculosis. Nontuberculous mycobacteria. (i) Mycobacterium avium complex. (ii) Mycobacterium kansasii. (iii) Mycobacterium malmoense. (iv) Mycobacterium xenopi. (v) Rapidly growing mycobacteria. Fungal Infections Aspergillosis. Zygomycosis. Histoplasmosis. Blastomycosis. Coccidioidomycosis. Paracoccidioidomycosis. Cryptococcosis. Penicillium. Pneumocystis jiroveci. Miscellaneous fungi. Parasites Echinococcus. Paragonimiasis. CAVITARY LUNG DISEASE IN SPECIFIC HOSTS Human Immunodeficiency Virus Hematologic Malignancy and Transplantation CONCLUSIONS ACKNOWLEDGMENTS REFERENCES
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Summary: A pulmonary cavity is a gas-filled area of the lung in the center of a nodule or area of consolidation and may be clinically observed by use of plain chest radiography or computed tomography. Cavities are present in a wide variety of infectious and noninfectious processes. This review discusses the differential diagnosis of pathological processes associated with lung cavities, focusing on infections associated with lung cavities. The goal is to provide the clinician and clinical microbiologist with an overview of the diseases most commonly associated with lung cavities, with attention to the epidemiology and clinical characteristics of the host.
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Cavities are frequent manifestations of a wide variety of pathological processes involving the lung. The presence of a cavity helps the clinician to focus the diagnostic evaluation, as some diseases are more commonly associated with cavities than others. In the case of infectious diseases, cavitation represents the outcome of complex interactions between host and pathogen. The focus of this review is to assist the clinician and clinical microbiologist in the evaluation of patients presenting with pulmonary cavities. We will broadly review the differential diagnosis of pulmonary cavities and specifically examine host-pathogen interactions associated with cavitation.
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A cavity has been defined in the radiology literature as (pathologically) "a gas-filled space within a zone of pulmonary consolidation or within a mass or nodule, produced by the expulsion of a necrotic part of the lesion via the bronchial tree" and (radiographically) "a lucency within a zone of pulmonary consolidation, a mass, or a nodule; hence, a lucent area within the lung that may or may not contain a fluid level and that is surrounded by a wall, usually of varied thickness" (368). In theory, one would like to distinguish a cavity from other air- or fluid-filled lung structures with different pathophysiologies, but in practice, this is not always possible. Some have tried to make this distinction by defining cysts as being air-containing spaces surrounded by a thin (4 mm or less) wall and cavities as being as air-containing spaces with walls that are at least 5 mm thick (322). Unfortunately, considerable overlap in etiology and pathophysiology exists between these two categories. For example, bronchogenic cysts are benign developmental abnormalities of the lung that usually appear to be homogeneous masses with the density of water. However, bronchogenic cysts may contain air and have been confused with more typically cavitary lesions such as lung abscesses, fungal infections, or tuberculosis (7). Other air-filled pulmonary lesions such as emphysematous bullae may also be radiographically indistinguishable from cavities (212). Therefore, for purposes of this discussion, a cavity will be defined as any radiographic opacity with an internal area of lucency, regardless of wall thickness.
A cavity is the result of any of a number of pathological processes including suppurative necrosis (e.g., pyogenic lung abscess), caseous necrosis (e.g., tuberculosis), ischemic necrosis (e.g., pulmonary infarction), cystic dilatation of lung structures (e.g., ball valve obstruction and Pneumocystis pneumonia), or displacement of lung tissue by cystic structures (e.g., Echinococcus) (204). In addition, malignant processes may cavitate because of treatment-related necrosis, internal cyst formation, or internal desquamation of tumor cells with subsequent liquefaction (92, 251). The likelihood that a given process will cavitate depends upon both host factors and the nature of the underlying pathogenic process. The prevalence of cavities among persons with a given process varies widely. In general, certain processes tend to form cavities more commonly than others. For example, Mycobacterium tuberculosis generally has the highest prevalence of cavities among persons with pulmonary disease of any infection, probably because this pathogen causes extensive caseous necrosis. In the case of M. tuberculosis, the tendency to form cavities is clearly advantageous to the propagation of the organism because cavities contain large numbers of organisms, which can then be efficiently aerosolized and transmitted to other susceptible hosts (129, 347). Other pathogens, such as Klebsiella pneumoniae, are associated with extensive pyogenic lung necrosis and frequent cavitation (254). This organism is also disproportionately represented among cases of pulmonary gangrene, in which there is extensive pulmonary necrosis and infarction, suggesting that the organism possesses pathogenic determinants that are more likely to lead to pulmonary necrosis and cavitation than other common causes of pulmonary infection, such as Streptococcus pneumoniae (296). The predilection to form necrotic cavities may be due to the priming of the inflammatory response by the concurrent aspiration of stomach acid (372) or factors specific to the organism, such as endotoxin (348). Unfortunately, there is no single common factor that differentiates organisms that are frequently associated with pulmonary cavitation from organisms that are rarely associated with pulmonary cavitation. However, as a general rule, organisms that cause subacute or chronic pulmonary infections (e.g., mycobacteria and fungi) seem to be more frequently associated with cavities than organisms that cause acute pulmonary infections (e.g., viruses and S. pneumoniae). This rule has many exceptions (e.g., necrotizing pneumonias associated with Staphylococcus aureus and K. pneumoniae).
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Plain chest radiography and computed tomography are the radiographic modalities most often used to image the chest. Ultrasound is a suboptimal modality for imaging the lung parenchyma because of poor sound transmission through the mostly air-filled lungs (237). Magnetic resonance imaging of the lung has been limited by motion artifact and relatively low spatial resolution (265), so this modality is not generally used to examine the lungs. Computed tomography is clearly more sensitive than plain chest radiography for the detection of pulmonary pathology, particularly in immunocompromised hosts. For example, one study of 61 patients at a single institution who had undergone bone marrow transplants for malignancies demonstrated that plain chest radiography was 58% sensitive in detecting pulmonary infection, compared to a sensitivity of 89% for computed tomography (P < 0.0001), with similar specificities for both modalities (328). Another study of 188 high-resolution computed tomography scans performed on 112 patients with febrile neutropenia and normal chest radiographs reported that 60% of the scans (112/188) had findings suggestive of pneumonia. However, these abnormal scan findings were not specific, as 46% of these 112 scans did not meet criteria for the diagnosis of pneumonia upon follow-up (150).
The radiographic appearance of cavitary lesions can sometimes be useful to differentiate among a broad spectrum of etiologies but should be combined with clinical and laboratory data to obtain an accurate diagnosis. One traditional method used to classify cavitary lesions is wall thickness. Cavitary lesions associated with specific diseases are frequently described as being "thick walled" or "thin walled," but exact definitions for these terms are often lacking. Of course, measured wall thickness will depend on the imaging technique used (plain radiography or computed tomography). Two studies examined the predictive utility of cavity wall thickness in solitary lung cavities as measured by plain radiography (399, 400). Those studies found that the measurement of the cavity wall thickness at its thickest section was most useful in predicting whether the cavity was of malignant versus nonmalignant etiology. Cavities with a maximum wall thickness of 4 mm or less were usually (30/32 [94% of the time]) caused by nonmalignant processes. Cavities with a maximum wall thickness of 5 to 15 mm were mixed, with 33/55 (60%) being nonmalignant and 22/55 (40%) being malignant cavities. Cavities with a maximum wall thickness of >15 mm were usually (35/39 [90%]) malignant. In those two studies, the location of the lesions and the presence of an air-fluid level did not correlate well with malignant versus nonmalignant etiology. Another small study compared the characteristics, as observed on computed tomography scans of the lung, of cavities caused by lung cancer and nonmalignant cavities associated with intracavitary aspergilloma (290). Cavities associated with lung cancer in that study had significantly thicker walls than cavities associated with aspergillomas (mean wall thickness of 5.8 mm for lung cancers and 2.6 mm for aspergillomas; P = 0.035), but there was significant overlap in wall thickness between lung cancers and aspergillomas. Furthermore, cavity wall thickening observed using computed tomography may be an early sign of the development of an intracavitary mycetoma (325), so wall thickness is at best an imperfect tool for discriminating between malignant and nonmalignant etiologies of pulmonary cavities. The use of cavity wall thickness to discriminate among infectious etiologies of pulmonary cavities is even more problematic. While some infections, such as Pneumocystis pneumonia, coccidioidomycosis, and Echinococcus, have been classically associated with thin-walled cavities, the absence of comparative studies with systematic, objective measurements of cavity wall thickness among infectious etiologies severely limits the use of cavity wall thickness as a diagnostic tool in discriminating among infectious causes of cavities.
While wall thickness alone has at best questionable utility in discriminating between malignant and nonmalignant etiologies of a pulmonary cavity, other radiographic characteristics may provide additional clues to the nature of the underlying disease. The presence of a cavity on computed tomography of the lung essentially ruled out a viral infection in a small study (n = 78) of immunocompromised patients with lung infection, but the etiologies of cavities among these patients were about equally divided among bacterial, mycobacterial, and fungal infections (115). Another study of 131 adults in South Korea with cavities on plain radiography examined radiographic factors associated with specific disease etiologies (404). In that study, in which 50% of subjects had active or prior mycobacterial lung disease (primarily tuberculosis), the presence of the largest cavity in the upper lobes suggested a mycobacterial etiology, while lesions confined to only one lobe and the presence of multiple enlarged mediastinal lymph nodes were associated with another etiology (about half of which [31/65] were malignant). Nonradiographic factors such as age of >50 years and a history of malignancy were also associated with nonmycobacterial etiology. Of note, cavity wall thickness did not differ between subjects with mycobacterial cavities and those with nonmycobacterial cavities in that study.
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One of the most important distinctions in the differential diagnosis of cavitary lung lesions is the distinction between malignant and nonmalignant etiologies. Primary lung cancer is a common disease, with 190,297 incident cases and 150,997 deaths reported in the United States in 2003 (370). Cavitation detected by plain radiography has been noted in 7 to 11% of primary lung cancers (58, 92, 247, 261), while cavitation detected by computed tomography has been reported for up to 22% of primary lung cancers; cavitation is more frequently found among cases of squamous cell carcinomas than other histological types (58, 247, 283). Furthermore, the presence of cavitation in a lung tumor has been associated with a worse prognosis (196). Other primary tumors in the lung, such as lymphoma and Kaposi's sarcoma, may also present with cavitary lesions, particularly among persons infected with human immunodeficiency virus (208, 235). In one series of human immunodeficiency virus-infected persons with primary pulmonary lymphoma, 5/12 subjects had cavitary lesions noted on computed tomography scans of the chest (305). Lymphomatoid granulomatosis, a rare malignant disorder associated with Epstein-Barr virus and clonal B-cell replication, frequently presents with pulmonary cavities and may be confused with lung abscess (240). Metastatic disease from other primary sites may also cavitate, but this occurs less frequently than in primary lung cancers: an estimated 4% of metastatic tumors have been noted to cavitate as detected by plain radiography (92). Interestingly, metastatic tumors of squamous cell origin are also more likely to cavitate than tumors of other origins, suggesting a common pathogenesis for cavitation among these tumors.
Complicating the diagnostic evaluation of cavitary lung lesions is the not-infrequent coexistence of pulmonary infection and malignancy. Multiple cases in which cavitary pulmonary lesions represent a combination of malignancy and an infectious pathogen have been reported. One prospective study based at a single center in Taiwan examined 22 patients with cavitary lung lesions, without evidence of postobstructive pneumonia, for whom ultrasound-guided transthoracic needle biopsy was performed (215). Nine pathogens were isolated from seven of the 22 patients, including K. pneumoniae (3 patients), Haemophilus influenzae (2 patients), Bifidobacterium (1 patient), Enterococcus faecium (1 patient), M. tuberculosis (1 patient), and Shewanella putrefaciens (1 patient). Additionally, multiple case reports described coexistent malignancy and infectious pathogens in cavitary lung lesions. In particular, primary lung cancer and tuberculosis are not infrequently encountered together, and either one can be responsible for cavitary lesions (190). The causal pathway for this association can go both ways: chronic inflammation and scarring caused by tuberculosis may contribute to the development of malignancy at the site, or immunosuppression associated with cancer and treatment may result in the reactivation of tuberculosis. Other mycobacterial or fungal pathogens can also coexist in malignant cavities; one report described concurrent Aspergillus, Mycobacterium xenopi, and lung cancer in a single patient (343).
Many autoimmune diseases can affect the lung, but cavitation is relatively uncommon in most of these diseases. The exception is Wegener's granulomatosis, an uncommon disorder in which cavitary lung disease is frequently encountered. Wegener's granulomatosis is a systemic vasculitis that almost always involves the upper or lower respiratory tract. Pulmonary nodules and infiltrates are a frequent manifestation of Wegener's granulomatosis in the lung, and cavitation may accompany both of these manifestations. One study of 77 persons with Wegener's granulomatosis found that 26/53 (49%) persons with pulmonary nodules had cavitation (on plain radiograph or computed tomography) within one or more nodules and that 7/41 (17%) persons with infiltrates had cavitation within an area of infiltrate (subjects could have both nodules and infiltrates, so numbers do not equal 77) (77). Another study found cavitary nodules in 7/19 (37%) subjects with Wegener's granulomatosis detected by plain chest radiographs (3). The clinical picture was frequently complicated by infection in these subjects, as 3/19 (16%) subjects had subsequent bacterial superinfection of a lung cavity. Pulmonary cavities have been observed by computed tomography in 35 to 50% of patients with Wegener's granulomatosis involving the lung (203, 213, 236).
Sarcoidosis is a relatively common inflammatory disorder of unknown etiology that frequently affects the lungs (19). Plain chest radiographic findings are often nonspecific; conventional and high-resolution computed tomography are better modalities for showing characteristic features of pulmonary sarcoidosis (266). Hilar and mediastinal lymphadenopathy are usually present, with or without concomitant parenchymal abnormalities. Lung nodules are frequently observed and tend to be distributed along the bronchovascular bundles, interlobular septa, major fissures, and subpleural regions (266). Cavitation occasionally occurs within these nodules; for example, one study demonstrated cavitation in 3/44 (6.8%) patients with pulmonary sarcoidosis (29). Additional findings by computed tomography include fibrosis (honeycomb, linear, or associated with bronchial distortion), pleural thickening, and ground-glass opacities (1, 29).
Pulmonary cavities are less frequently encountered in other autoimmune diseases. Given that most patients with these diseases are treated with potent immunosuppressive agents, infectious etiologies for cavitary lesions should be aggressively investigated. However, cavitary lung lesions have been reported as being rare consequences of many autoimmune diseases. For example, patients with ankylosing spondylitis frequently (50 to 85%) have pulmonary abnormalities detectable by high-resolution computed tomography (45, 99, 330, 369), although a much smaller proportion have abnormal plain chest radiographs (1.3% in one large series) (319). Relatively common findings among patients with ankylosing spondylitis-associated lung abnormalities are apical fibrosis and bulla formation, both of which may appear radiographically as cavitation. Cavitation detected only by computed tomography in these patients is of uncertain clinical significance, but when cavities are visible by plain radiography, the etiology is commonly infection. For example, of 2,080 patients with ankylosing spondylitis in one series, 28 (1.3%) had lung abnormalities detected by plain radiography, most commonly apical fibrobullous lesions (319). Of these 28 patients, 7 had infectious etiologies for cavitary lesions: 5 had aspergillomas, and 2 had nontuberculous mycobacterial infections. Similarly, cavitary lesions in patients with systemic lupus erythematosus have also been reported, but most of these lesions represent infection. One small series found six patients with cavitary lung lesions among a population of 798 patients with systemic lupus erythematosus seen at one center; four of the six had infectious etiologies for the cavities (two mixed bacterial infections with gram-positive and gram-negative organisms, one Pseudomonas aeruginosa infection, and one Aspergillus fumigatus infection), one cavity possibly represented pulmonary infarction, and only one patient had cavities that were likely attributable to lupus (386). Rheumatoid arthritis is also commonly associated with pulmonary abnormalities, but cavities due primarily to rheumatoid arthritis are rare. Lung cavities in patients with rheumatoid arthritis often represent infection or carcinoma, so aggressive diagnostic evaluation is warranted for new cavitary lesions in these patients (172). In rare cases, rheumatoid nodules may appear in the lung and cavitate, presumably due to ongoing vasculitis with ischemic necrosis (185, 408). Primary amyloidosis is another rare autoimmune cause of pulmonary cavities (354).
Various other noninfectious disease processes have been associated with lung cavities. Of these, the most common is pulmonary embolism. While pulmonary embolism is usually associated with nonspecific radiographic changes or even a normal chest radiograph (402), pulmonary infarction and necrosis may result in a cavitary lesion. Pulmonary embolism usually occurs when a clot forms in a lower extremity or the pelvis and subsequently breaks off and lodges in a pulmonary artery. However, pulmonary embolism may also be caused by tumor, fat (associated with trauma), air bubbles, septic emboli (see below), or injected foreign substances such as talc (145). Pulmonary infarction has been estimated to occur in 10 to 15% of cases of pulmonary embolism in older studies (217, 260); more recent studies using computed tomography noted infarction in up to 32% of patients with pulmonary embolism (148). In turn, cavitation has been observed on plain radiographs in 2.7 to 7% of patients with pulmonary infarction (67, 217) and in up to 32% of patients with pulmonary infarction detected by computed tomography (148). The lesions tend to be located in the lung periphery but may be located in any lobe, and there is nothing specific about the distribution or appearance of the cavities (391). Immunocompromised hosts are often at risk for pulmonary embolism due to underlying malignancy, surgical procedures, and immobility, so aseptic pulmonary embolism may be an underappreciated cause of lung cavities in these hosts (258). Cavities associated with venous thromboembolism may be observed 2 to 63 days after the embolic event and, in most cases, are initially aseptic (217, 391). However, superinfection of necrotic lung cavities is common, occurring in approximately one-half of cases (391); gram-negative rods are frequently implicated in these superinfections. Furthermore, superinfection with Clostridium species has been particularly associated with pulmonary embolism and usually causes a necrotizing, cavitary pneumonia that may further complicate the clinical picture (38).
A less common disorder associated with lung cavities is bronchiolitis obliterans organizing pneumonia, also called cryptogenic organizing pneumonia when there is no underlying etiology. This disorder, which is a pathological diagnosis, may be triggered by drug or toxin exposure, autoimmune diseases, viral infections, or radiation injury but is most often idiopathic (272). Patients with bronchiolitis obliterans organizing pneumonia usually present with fever, cough, weight loss, and dyspnea over weeks to months, similar to many infectious diseases associated with lung cavities (76). The most common computed tomography appearance of this disorder is patchy consolidation, often accompanied by ground-glass opacities and nodules (214). Cavitation has been reported in 0 to 6% of cases, varying with the series and the imaging modality (76, 101, 214). Unfortunately, none of the clinical or radiographic manifestations of this disorder are specific, and diagnosis must be made by lung biopsy (101).
Another rare disease associated with lung cavities is pulmonary Langerhans' cell histiocytosis. The disease almost exclusively (over 90%) afflicts smokers, with a peak age of onset of between 20 and 40 years (357). Clinical presentation varies, but symptoms generally include months of dry cough, fever, night sweats, and weight loss (357). Thin-walled cystic cavities are the usual radiographic manifestation, observed in over 50% of patients by either plain chest radiography or computed tomography scans, but thicker-walled cavities are also commonly observed (30, 255). Diagnosis may be suggested by radiographic findings but must be definitively made by lung biopsy (357).
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Infections caused by commonly encountered gram-positive or gram-negative bacteria may cause pulmonary cavities by one of two mechanisms. First, organisms may enter the respiratory cavity via the oropharynx/upper airway, bypass host defenses, and cause either a necrotizing pneumonia or lung abscess. Alternatively, organisms may enter the lung via the bloodstream, often in association with fibrin and platelets as septic pulmonary emboli. The radiographic appearance and microbiological etiologies of these two mechanisms are distinct and will be discussed separately.
Necrotizing pneumonias and lung abscesses. Lung cavities have not typically been associated with community-acquired pneumonia, but occasional cases of cavitary pneumonia due to Streptococcus pneumoniae or Haemophilus influenzae have been reported (312, 405). Of course, the prevalence of cavities among patients with community-acquired pneumonia depends on the imaging technique used; significantly more cavities are detected with computed tomography than with plain radiography. For example, among 17 children with severe pneumonia and cavitation detected by computed tomography, only 10 (59%) had cavities noted on plain radiographs (96). One series of 105 adults hospitalized with pneumococcal pneumonia reported that no patient had cavities detected by plain radiography (332); computed tomography was not routinely performed. On the other hand, a study of another series of 35 patients (43% with human immunodeficiency virus) with pneumococcal pneumonia reported that 7 (20%) had lung cavities detectable on computed tomography scans (345). Cavitation is more frequently reported among patients with concurrent S. pneumoniae pneumonia and bacteremia, which may reflect the greater severity of disease among bacteremic patients (184). Similarly, a recent study of 75 children with empyema or parapneumonic effusion reported that 15 (20%) had associated cavitary lung disease, 13 of whom had evidence of S. pneumoniae infection (304). Because S. pneumoniae and H. influenzae are such common causes of pneumonia, these pathogens may cause a significant fraction of cavitary pneumonias, even though cavitation is relatively rare with these pathogens. This point is illustrated by a small case series of nine children hospitalized with cavitary pneumonia (as seen by computed tomography); the etiology was S. pneumoniae in three of nine patients and S. pneumoniae and H. influenzae in one patient (153).
Klebsiella pneumoniae is a common cause of severe, necrotizing pneumonia. While older literature described alcoholism and smoking as important risk factors for community-acquired Klebsiella pneumonia, more recent studies demonstrate that a growing proportion of patients are immunocompromised and acquire infection in the hospital (43, 199). It has been a relatively common cause of community-acquired pneumonia (7.5% of all cases in North America) (152) and is particularly prevalent among patients with severe community-acquired pneumonia (22% of cases in one series) (286). K. pneumoniae pneumonia is frequently complicated by lung abscess, which generally appears as one or more cavities. In older series, 0 to 40% of patients developed lung abscess as noted on plain chest radiographs (43). A more recent series of 23 patients with K. pneumoniae pneumonia reported multiple small cavities ranging from 1 mm to 3 cm in diameter in 11 (48%) patients by computed tomography. Lung abscess due to K. pneumoniae can progress to destroy an entire section of a lung, a condition known as massive pulmonary gangrene. Pulmonary gangrene is a rare condition, but over one-half of cases are attributable to K. pneumoniae (296). Radiographically, this condition starts with lung consolidation, followed by the development of multiple small cavities that coalesce into one large cavity (84).
Staphylococcus aureus is an emerging cause of cavitary pneumonia (Fig. 1). In older series, S. aureus accounted for about 1% of community-acquired pneumonias (226). Plain chest radiographs frequently demonstrated cavitation in both adults and children. In one series of 26 adults and 8 children with community-acquired S. aureus pneumonia, 7 of the adults (27%) and 2 of the children (25%) had cavities detected by plain chest radiographs, while 4 adults (15%) and 3 children (38%) had pneumatoceles, defined as thin-walled cystic structures (227). More recently, community-acquired methicillin-resistant S. aureus, which usually possesses the Panton-Valentine leukocidin virulence factor, has become an emerging cause of severe pneumonia. In contrast to classic staphylococcal pneumonia, which typically afflicted relatively debilitated patients in the nosocomial or health care-associated setting, community-acquired methicillin-resistant S. aureus pneumonia frequently affects immunocompetent hosts without significant prior exposure to the health care system. Pneumonia in these patients is frequently preceded by extrapulmonary staphylococcal infection, particularly skin infection. Community-acquired staphylococcal pneumonia is frequently severe. In one recent series of 50 patients, 78% required intubation and 42% had pulmonary hemorrhage, and overall in-hospital mortality was 56%. On plain radiographs, cavitation was noted in 12% of cases; multilobar consolidation (79%) and pleural effusions were more common (50%) (124).
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FIG. 1. Sequelae of severe Staphylococcus aureus pneumonia in a patient with multiple other comorbidities. The left panel illustrates the plain chest radiographic appearance, with multiple areas of fibrosis and a residual cavity in the medial right upper lobe. The right panel shows the same cavity using computed tomography.
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Septic pulmonary emboli. Septic pulmonary emboli, although relatively rare, are important considerations in the differential diagnosis of cavitary lung lesions. Septic emboli typically appear as nodules located in the lung periphery, although wedge-shaped peripheral lesions and infiltrates are also seen (166). Cavitation is seen in 23 to 47% of cases using plain radiography and in up to 85% of cases using computed tomography (73, 158, 202). The presence of a "feeding vessel" sign, in which a distinct vessel is seen leading to the center of a pulmonary nodule, suggests the diagnosis of septic embolus, but the specificity of this finding has been called into question (93). In older series, septic emboli were associated primarily with intravenous drug use and septic thrombophlebitis either in the pelvis or in the internal jugular vein (Lemierre syndrome) (228, 281). However, the etiologies of septic pulmonary embolism in more recent series are dominated by infected intravascular prosthetic material such as intravascular catheters, pacemaker wires, and right-sided prosthetic heart valves (73). Septic pulmonary emboli associated with intravenous drug use are caused predominantly by S. aureus (46, 281). Conversely, septic emboli associated with intravascular prosthetic material have been associated with a number of pathogens. For example, in a recent series at the Mayo Clinic, 7/14 cases of septic emboli were associated with intravascular prosthetic material (three central venous catheters, two prosthetic pulmonic valves, and two pacemakers) (73). The microbiological etiologies among patients with intravascular prosthetic material included three cases of S. aureus, two cases of coagulase-negative staphylococci, one case of S. pneumoniae, and one polymicrobial infection (coagulase-negative staphylococcus, Corynebacterium, and Klebsiella oxytoca). Septic thrombophlebitis of the internal jugular vein may also be catheter associated but has classically been associated with oropharyngeal infection and is termed Lemierre syndrome. Lemierre syndrome generally afflicts young persons in their late teens and early twenties but may also affect children and older persons (57, 73, 127). Most affected patients are immunocompetent and previously healthy. The majority of cases of Lemierre syndrome are associated with Fusobacterium necrophorum (71.6% in one large series) (57), but many are mixed infections composed of anaerobes normally present in the oral cavity. Almost all patients with Lemierre syndrome have pulmonary manifestations, including infiltrates and pleural effusions; cavities were seen in 31% of patients in one series (57). Apart from Lemierre syndrome, community-acquired methicillin-resistant S. aureus is increasingly associated with septic pulmonary emboli, particularly among children with bone and joint infections (130). Among profoundly immunocompromised persons, septic emboli due to nontyphoid Salmonella strains have been described. Persistent and recurrent Salmonella bacteremias, often with extraintestinal manifestations, are not rare among persons with advanced human immunodeficiency virus (131) and have been reported for other immunosuppressed hosts as well (5, 48). Pulmonary lesions have been reported in up to 35% of patients with advanced human immunodeficiency virus and Salmonella bacteremia (usually S. enterica serovar Enteritidis or S. enterica serovar Typhimurium), and in one report, 70% of patients with pulmonary manifestations of Salmonella had cavities detected by plain chest radiographs (44).
Actinomycosis. Actinomyces species are gram-positive rods that normally colonize the human digestive tract. Approximately 50% of disease caused by actinomycosis is cervicofacial, 20% involves the abdomen or pelvis, and 15% is pulmonary (70, 225). Actinomyces israelii is the most commonly isolated pathogen in all of these disease manifestations, but other species (Actinomyces naeslundi, A. odontolyticus, A. viscosus, A. meyeri, and A. gerencseriae) are also frequently implicated in disease (225). A. meyeri, in particular, may have a greater propensity to cause pulmonary disease as well as to disseminate (11). Alcoholism and poor oral hygiene have been associated with Actinomyces infection (11, 22). Pulmonary actinomycosis may occasionally result from the contiguous spread of infection from the neck but is usually caused by aspiration of saliva or other material containing Actinomyces (70). This aspiration results in pneumonitis followed by local necrosis, fibrosis, and cavitation (33). Not surprisingly, many Actinomyces infections are polymicrobial and include other components of the oral flora such as Eikenella corrodens, streptococci, Actinobacillus, or Fusobacterium (155). Pulmonary actinomycosis generally progresses slowly with nonspecific symptoms including nonproductive cough, low-grade fever, and malaise (341). The insidious nature of the disease, its proclivity to appear as a mass lesion on chest radiography, and the occasional presence of concurrent extrapulmonary disease make it easily confused with malignancy (68, 356). Cavitary lesions are common but may be visible only by computed tomography. For example, two recent series demonstrated cavitary lesions on plain radiographs in 0 and 13% of cases, respectively, but cavities were detected by computed tomography in 62% and 75% of cases (55, 206). Associated pleural thickening or effusion is common, and as the infection erodes through tissue planes, empyema necessitatis with drainage through the chest wall or into other spaces may complicate pulmonary actinomycosis (33, 113).
Nocardia. Nocardia species are gram-positive, weakly acid-fast organisms naturally found in the soil. Infection is presumably acquired via either inhalation or direct inoculation due to trauma. Over 50 species of Nocardia have been identified, with about 16 implicated in human infection. Traditionally, most isolates causing human disease were identified as being Nocardia asteroides, but molecular taxonomy has demonstrated that many of these isolates were misidentified, so the relationships between species and disease manifestations are changing rapidly (35). Pulmonary disease is the most common presentation of Nocardia infection and accounts for over one-half of reported cases in most series (105, 267, 301). The most common risk factor for pulmonary nocardiosis is underlying lung disease such as asthma, bronchiectasis, or chronic obstructive pulmonary disease (105, 159, 301). Persons with systemic immunodeficiencies associated with cancer chemotherapy, human immunodeficiency virus, organ transplant, and long-term corticosteroid use are also at risk for pulmonary Nocardia infection, but approximately 15% of patients will have no underlying disorder (159, 234). Disease onset is usually subacute, and symptoms including fever, cough, and dyspnea are usually present for several days to several weeks before diagnosis (244). The manifestations of Nocardia infection detected by plain chest radiography include multifocal consolidation, irregular masses, single or multiple nodules, and pleural effusions (106). Cavitation results from tissue necrosis and abscess formation and may be observed on plain radiographs in 38 to 62% of cases (106, 273). Computed tomography frequently reveals multiple nodules and pleural involvement, with cavitation in up to 80% of cases (407). Cavitation may be more frequently observed among persons with advanced human immunodeficiency virus than among other hosts (36). Nocardia infection may be associated with mortality rates as high as 30 to 40%, particularly among hosts with severe immune compromise (234, 244).
Melioidosis. Melioidosis, the term given to disease caused by the gram-negative rod Burkholderia pseudomallei, can affect any organ system but most commonly affects the lung (165). It is endemic in tropical areas, especially Southeast Asia and Northern Australia, although disease in rural areas in South and Central America is probably underrecognized (164). Infection usually occurs through contact with or ingestion or inhalation of contaminated soil or water. Independent risk factors for melioidosis include occupational exposure (such as from rice farming), diabetes, chronic renal disease, and thalassemia (353). Pulmonary disease can present in an acute or chronic form and either be localized or part of disseminated infection (165). Furthermore, infection can be latent and reactivate many years after exposure, such as is observed in Vietnam war veterans who develop disease many years after returning to the United States (198). Acute pulmonary disease presents as high fever, chills, coughing, chest pain, and dyspnea. Plain radiographs most commonly demonstrate alveolar or nodular infiltrates, and cavitation is relatively common (26% in one series of 183 patients in Northern Thailand) (91). Conversely, subacute or chronic pulmonary disease usually presents radiographically with nodular and/or alveolar infiltrates accompanied by cavities; 50% of nonbacteremic patients with subacute pulmonary disease and 68% of nonbacteremic patients with chronic pulmonary disease had cavities detected on plain chest radiographs in the same Thai series. Patients with subacute/chronic disease and concurrent bacteremia tend to have a lower prevalence of cavities (0 to 29%) than patients with subacute/chronic melioidosis without bacteremia (60%) (91, 264).
Rhodococcus. Rhodococcus equi is a gram-positive coccobacillus commonly isolated from soil, especially farm soil that has been contaminated with horse manure (355). The organism is a common cause of pneumonia in young horses and is shed in the feces of mares (138). Exposure to livestock is a recognized risk factor for infection, although many patients will not have this exposure (181, 205). Infection occurs primarily through inhalation (274), although infection may also be acquired by ingestion or traumatic inoculation (376). R. equi is increasingly being recognized as a human pathogen, particularly in those with advanced human immunodeficiency virus infection (CD4+ T-lymphocyte count of <200 cells/mm3) (133, 385), hematologic malignancies (223), and use of chronic corticosteroids and other immunosuppressive agents (120) and in recipients of solid-organ and stem cell transplants (79, 205, 268, 297). Depending on the series, up to 10 to 15% of cases occur in immunocompetent individuals (118, 181, 376).
Pulmonary disease is the most common manifestation of R. equi in immunocompromised patients, occurring in approximately 80% of cases (268, 270, 364, 376) (Fig. 2). Immunocompetent patients are more likely to have extrapulmonary disease, with approximately 40% of reported cases of R. equi disease in immunocompetent hosts affecting the lungs (181). Pulmonary R. equi generally presents with insidious onset of fever, dyspnea, cough (frequently nonproductive), and pleuritic chest pain (181, 270). Chest radiographs are usually abnormal, most frequently demonstrating dense infiltrates with or without upper lobe cavitation (270, 364). Cavitation detected by plain chest radiography has been observed in 63% of immunocompetent patients with pulmonary R. equi infection (181) and in 41% to 77% of human immunodeficiency virus-infected patients (94, 270, 364). One small (n = 5) study demonstrated cavitation in all patients using computed tomography (233), while another study (n = 9) demonstrated cavitation in 44% of patients using plain radiography and 67% of patients using computed tomography (390). Nodular infiltrates, pleural effusion, and mediastinal lymphadenopathy are other common radiographic features (233, 270, 390). A majority of immunocompromised patients with pulmonary disease will have concurrent bacteremia, so routine blood cultures are high yield in this population (94, 364).
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FIG. 2. Left lower lobe cavitary pneumonia due to Rhodococcus equi with concurrent bacteremia in a patient with advanced human immunodeficiency virus.
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Mycobacterium tuberculosis. Mycobacterium tuberculosis is classically associated with cavitary pulmonary disease (Fig. 3). Although tuberculosis case rates have declined in many developed countries, the human immunodeficiency virus epidemic has led to a tremendous increase in tuberculosis cases in the developing world, particularly in sub-Saharan Africa (75). In addition to human immunodeficiency virus infection, other risk factors for tuberculosis include exposure-related factors such as birth in a country where tuberculosis is endemic (294) and immunologic deficits that increase the risk of progression from latent to active tuberculosis, such as diabetes (188), hematologic and head and neck malignancies (175), organ transplantation, corticosteroid use, and tumor necrosis factor antagonist use (8, 180). Pulmonary tuberculosis generally presents subacutely, with weeks to months of productive cough, fever, night sweats, weight loss, and, occasionally, hemoptysis. The chest radiograph typically reveals pulmonary infiltrates in the apical and posterior segments of the upper lobe or the superior segment of the lower lobe, often associated with cavitation (248, 371). The prevalence of cavities on plain radiographs varies widely by series, but most series report cavitation in 30 to 50% of patients. Multiple cavities are often present and frequently occur in areas of consolidation (142, 238, 401). Cavities can vary widely in size and have been reported to have both thick and thin walls (142, 371, 393). The presence of cavitation is associated with a greater degree of infectiousness, likely due to higher organism burden (315). Supporting the association between cavitation and organism burden, cavitary disease is independently associated with increased time for acid-fast smears and cultures to become negative in patients receiving tuberculosis therapy as well as an increased risk of relapse after treatment completion (21). Similar to other cavitary lung diseases, computed tomography is more sensitive than plain radiography for the detection of tuberculous cavities (162, 238). Due in part to this increased sensitivity, the presence of cavitation detected by computed tomography scans does not necessarily carry the same clinical, prognostic, and public health impact as the presence of cavitation detected by plain radiography.
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FIG. 3. Extensive cavitary lung disease due to Mycobacterium tuberculosis visualized by plain chest radiography (left) and computed tomography (right). Note the typical upper lobe predominance and extensive fibronodular infiltrates.
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Nontuberculous mycobacteria. Over 120 species of nontuberculous mycobacteria, also known as mycobacteria other than tuberculosis or potentially pathogenic environmental mycobacteria, have been described (365). Unlike organisms belonging to the M. tuberculosis complex, nontuberculous mycobacteria are acquired from environmental exposure and are not transmissible from person to person (110). Nontuberculous mycobacteria are present in soil, water, and dust, so pulmonary infection occurs presumably via inhalation of infectious aerosols (191, 291). Pulmonary disease due to nontuberculous mycobacteria frequently presents with nonspecific symptoms such as chronic cough, fatigue, and weight loss. The syndromes caused by pulmonary infection by nontuberculous mycobacteria overlap significantly with each other and with tuberculosis, making the microbiological data crucial for obtaining the correct diagnosis. Furthermore, the isolation of any nontuberculous mycobacterium from a respiratory specimen may represent laboratory contamination, colonization, or true disease, so standardized diagnostic criteria are available to assist clinicians (136). In the next section, we will discuss some of the nontuberculous mycobacteria most commonly associated with pulmonary disease, recognizing that our understanding of species designations and their associations with particular disease syndromes is evolving.
(i) Mycobacterium avium complex. Organisms belonging to the Mycobacterium avium complex are the nontuberculous mycobacteria most frequently implicated in pulmonary disease in the United States (136). The M. avium complex includes two major species, M. avium and M. intracellulare, as well as a third group of organisms, many of which do not belong to named species (366). Early reports described a disease very similar to tuberculosis, manifesting as apical fibrocavitary disease in middle-aged men with a significant smoking history and underlying lung disease (321) (Fig. 4). Alcoholism and prior tuberculosis disease are also putative risk factors. Most patients complain of chronic cough; constitutional symptoms are not usually present until late in the disease. As the diagnosis was often made, in part, on the basis of cavitation, plain radiographs showed cavities in up to 88% of cases (59). In more recent years, the spectrum of lung disease caused by the M. avium complex has expanded. Probably the most common manifestation of M. avium complex pulmonary disease is the nodular/bronchiectatic form. This entity usually afflicts thin women over 50 years of age who have otherwise normal immune systems and no previous diagnosis of lung disease (302). Gastroesophageal reflux seems to be a significant risk factor, and aspiration may play a significant role in the pathogenesis of the disease (195, 360). Cavitation is rarely visible on plain chest radiographs, but computed tomography scans reveal cavities in up to 62% of cases (224, 395). Other typical radiographic features detected by computed tomography include nodules with associated bronchiectasis, particularly in the lingula and/or right middle lobe (64, 154, 224). Obtaining a microbiological diagnosis from sputum specimens is often difficult in patients with nodular/bronchiectatic M. avium complex pulmonary disease; in one series, 45% of patients required bronchoscopy or lung biopsy to obtain a diagnosis (157). Computed tomography scans may be helpful to refine the diagnostic approach for patients with suspected M. avium complex pulmonary disease, as cavitation detected by computed tomography has been positively correlated with the likelihood of obtaining a positive sputum culture (224). The M. avium complex can also cause solitary pulmonary nodules and hypersensitivity pneumonitis, but these entities are not associated with cavitation (147, 186, 194).
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FIG. 4. Mycobacterium avium complex of the fibrocavitary type in a 52-year-old woman with chronic obstructive pulmonary disease visualized by plain radiography (left) and computed tomography (right).
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FIG. 5. Mycobacterium kansasii pulmonary disease presenting as 8 months of hemoptysis without any systemic symptoms in a 16-year-old girl. The thin-walled cavities (in the right apex) are relatively subtle on plain chest radiography (left) but are readily apparent by computed tomography, as is the accompanying bronchiectasis (right).
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(iv) Mycobacterium xenopi. Mycobacterium xenopi is an emerging pulmonary pathogen in Canada, the northeastern United States, the United Kingdom, and Europe (95, 231, 338, 359). In particular, M. xenopi is increasingly being isolated from respiratory specimens obtained from persons with human immunodeficiency virus infection, but the clinical significance of these isolates is questionable (95, 183, 231). Cavities are commonly (up to 81%) observed on plain radiography in patients without human immunodeficiency virus infection (169) but are rare among persons with M. xenopi pulmonary disease and human immunodeficiency virus coinfection. For example, in one series of 35 patients with advanced human immunodeficiency virus infection and nontuberculous mycobacterial infection (23 of whom had M. xenopi), only 1 patient (3%) had a cavity detected by plain chest radiography (100). As with M. kansasii, more modern series are recognizing that nodules and bronchiectasis are significant radiographic components of M. xenopi disease. In a recent small (n = 9) study of patients with M. xenopi pulmonary disease who had computed tomography performed, all nine had bronchiectasis, eight of nine (89%) had nodules, and five of nine (56%) had cavitation.
(v) Rapidly growing mycobacteria. Rapidly growing mycobacteria are increasingly being recognized as a cause of chronic lung disease. Among the rapidly growing mycobacteria, Mycobacterium abscessus is the predominant pathogen, accounting for 82% of infections in one large series of patients with pulmonary disease due to rapidly growing mycobacteria (n = 154); other pathogens in this series included M. fortuitum and M. chelonae (137). Most affected patients are nonsmoking Caucasian women that are middle-aged or older. Gastroesophageal reflux and/or achalasia have been implicated as being risk factors for pulmonary disease with these organisms (137, 141, 195). Due to its prevalence, the radiographic findings of M. abscessus have been best described. Cavitary lesions, often visible only by computed tomography, have been reported in 16 to 42% of patients (137, 146). Other findings commonly include the tree-in-bud pattern (branching nodular opacities), bronchiectasis, well-defined nodules, consolidation, and cavities. While cavitation is less common among patients with M. abscessus than among those with M. avium pulmonary disease, the radiographic findings overlap and are not clinically useful to identify the causative organism (64).
Aspergillosis. Aspergillus species are environmental molds that cause a wide range of pulmonary disease in humans. Pulmonary disease is most commonly caused by Aspergillus fumigatus, although it can be caused by other species such as A. flavus, A. niger, and A. terreus, and can manifest as one of four distinct clinical entities, ordered by increasing pathogenicity and tissue invasion: (i) allergic bronchopulmonary aspergillosis, which afflicts patients with long-standing asthma; (ii) aspergilloma, which afflicts primarily patients with preexisting lung cavities; (iii) chronic necrotizing aspergillosis or semi-invasive aspergillosis, which afflicts patients with a history of chronic lung disease; and (iv) invasive aspergillosis, which afflicts immunocompromised and critically ill hosts (342). Allergic bronchopulmonary aspergillosis is not generally associated with lung cavitation and will not be discussed further, but the other three manifestations of Aspergillus are all associated with pulmonary cavities.
An aspergilloma, also referred to as a mycetoma or fungus ball, represents growth of aspergillus (usually A. fumigatus) (299) within a preexisting lung cavity (Fig. 6). Classically, the most common cause of the cavity was pulmonary tuberculosis, and one older study reported radiographic evidence of aspergilloma formation in 11% of 544 patients with tuberculous pulmonary cavities (309). In areas where tuberculosis is endemic, tuberculosis is still the most common condition predisposing subjects to aspergilloma formation (6). However, any illness that causes a chronic, nonresolving pulmonary cavity produces an environment conducive to aspergilloma formation, and aspergillomas have been reported in association with most of the disease entities discussed in the present review. Radiographically, an aspergilloma appears as a rounded opacity within a previously existing cavity; computed tomography can more accurately delineate the mass and surrounding air crescent than plain radiography (6). Differentiating aspergilloma from malignancy is a significant issue, as there is considerable overlap in the appearances of the two conditions. Enhancement of the mass on computed tomography suggests malignancy, while adjacent bronchiectasis and a dependent location are more typical of aspergilloma (290). Other aspergilloma manifestations include thickening of the cavity wall, a new air-fluid level within the cavity, or complete opacification of a previously air-filled cavity (12). Patients with aspergilloma are frequently asymptomatic, but the most common symptomatic presentation is hemoptysis (307, 309). Aspergillomas may grow or shrink over time, and a small percentage (5 to 10%) may spontaneously resolve (309).
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FIG. 6. Aspergilloma (round mass in the left upper lobe) visualized by computed tomography in a young man. The etiology of the underlying cavity was unknown in this case.
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Invasive pulmonary aspergillosis afflicts primarily severely immunocompromised patients, especially those with hematological malignancies, bone marrow transplant recipients, and those with long-term immunosuppressive or corticosteroid use (252, 342). Persons who have received allogeneic bone marrow transplants have the highest rates of invasive aspergillosis. Among recipients of solid-organ transplants, lung transplant recipients have the highest risk of invasive aspergillosis (140, 257). However, invasive aspergillosis among critically ill persons without malignancy or other immune compromise is becoming an increasingly recognized entity (243). Invasive aspergillosis has also been reported among persons with advanced human immunodeficiency virus, usually in association with neutropenia and corticosteroid treatment (216, 275) Symptoms include fever, dyspnea, nonproductive cough, and chest pain; many patients with prolonged neutropenia will present with persistent fever despite broad-spectrum antibacterial therapy (342). Early in disease, plain radiographs usually demonstrate consolidation or nodules with no evidence of cavitation. Computed tomography scanning is more useful for early diagnosis than plain radiography (Fig. 7). Specifically, the presence of a "halo sign," defined as a nodule surrounded by a zone of ground-glass attenuation, is reasonably sensitive (70 to 80%) and specific (60 to 98%) for invasive aspergillosis in high-risk patients (26, 156, 292, 308). Cavitation generally occurs later in the course of the disease (1 to 2 weeks after the appearance of the halo sign) and is often noted during recovery from neutropenia in previously neutropenic patients (83, 122, 201). The onset of cavitation is heralded by the so-called "air crescent sign," defined as crescents of air surrounding nodular lesions; further necrosis due to fungal angioinvasion and resultant ischemia results in progressive cavity formation in up to 63% of patients (4, 26, 156, 292).
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FIG. 7. Invasive aspergillosis in a 52-year-old man with systemic lupus erythematosus who had been on chronic high-dose corticosteroids and azathioprine. The plain chest radiograph (left) is suggestive of septic pulmonary emboli, with multiple large nodules bilaterally, at least two of which contain central cavities. The right panel demonstrates one of these thick-walled cavities as seen by computed tomography. This patient also had Aspergillus in the brain, which is a common site of metastatic spread in immunocompromised hosts.
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Histoplasmosis. The dimorphic fungus Histoplasma capsulatum is one of the more common endemic mycoses, as it is found in soil worldwide. Disease due to H. capsulatum is highly endemic in the Midwestern United States, but low-level endemic disease has also been reported in Mexico, Central America, and South America as well as in parts of Southeast Asia (62, 121, 207, 331, 383). Infection occurs when H. capsulatum conidia are inhaled, so activities such as mining, speleology, construction, and agriculture are associated with an increased risk of disease (178). Pulmonary infection with H. capsulatum is generally asymptomatic; only an estimated 0.5% of infected persons will develop symptomatic disease (388). Most clinically apparent acute infections are mild and self-limited and have nonspecific symptoms such as fever, chills, anorexia, cough, and chest pain, often accompanied by intrathoracic lymphadenopathy (388). Cavitary disease may be a manifestation of acute infection or chronic disease. In one large outbreak in Indianapolis, Indiana, 62/741 (8.4%) patients with symptomatic acute histoplasmosis had cavitation detected by plain chest radiographs. Compared with patients without cavitation, patients with cavitation were more likely to be older, white males with a prior history of chronic lung disease or immunosuppression (389). Cavities were most commonly located in the upper lobes and accompanied by infiltrates and pleural thickening. Another review of cases of pulmonary histoplasmosis presenting to the Mayo Clinic in Rochester, Minnesota, from 1964 to 1974 reported that only 5/269 (1.8%) patients had pulmonary cavitation (72). The chronicity of infection in the Mayo Clinic series was not reported. Among patients with chronic pulmonary histoplasmosis, cavitation was present on plain radiographs in 33 to 68% of cases (61, 182, 331). Disseminated histoplasmosis, which usually occurs in immunocompromised hosts (particularly persons with advanced human immunodeficiency virus infection), usually does not cause cavitary lung lesions. For example, in one series of patients with advanced human immunodeficiency virus and disseminated histoplasmosis, 23/50 (46%) had abnormal plain chest radiographs with nodules or infiltrates, but none had cavities (71).
Blastomycosis. Pulmonary blastomycosis, caused by the dimorphic fungus Blastomyces dermatitidis, is endemic to north central United States and southern central Canada, with somewhat lower-level endemicity in the southeastern United States (62, 177). Although it is referred to as North American blastomycosis, endemic disease has been reported in Africa, India, South America, and Israel (42). Similar to the other endemic mycoses, blastomycosis is found in the soil and causes infection when the soil is disturbed and the fungus is inhaled (327). Blastomycosis most commonly afflicts immunocompetent hosts, although persons with diabetes mellitus seem to be disproportionately affected (39, 78, 374). The lungs are the most common site of disease, with pulmonary involvement reported in 60 to 93% of cases (78, 177, 374). A relatively high proportion (17 to 26%) of patients with pulmonary disease will have concurrent extrapulmonary involvement, particularly cutaneous or bone infection (78, 293, 374). The spectrum of pulmonary disease ranges from acute and often self-limited disease to chronic disease, which may last for years. Acute illness frequently presents with a relatively sudden onset of fever and cough, accompanied by alveolar infiltrates and occasionally nodular densities detected by plain chest